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water from dry riverbeds water from dry riverbeds Document Transcript

  • Water from Dry RiverbedsHow dry and sandy riverbeds can be turned into water sourcesby hand-dug wells, subsurface dams, weirs and sand damsWomen drawing water from an Water being drawn from a hand-dugunlined waterhole in a sandy riverbed. well on a riverbank upstream of a subsurface dam.A weir constructed across Talek river in Maasai Mara was extended to be a sand dam toprovide domestic water for a tourist camp. The lush vegetation seen upstream is due tothe weir having raised the water table in the riverbanks. Erik Nissen-Petersen for Danish International Development Assistance (Danida) 2006
  • Technical handbooks in this series :Titles Contents1 Water for rural communities Lessons learnt from Kitui pilot projects2 Water supply by rural builders Procedures for being rural contractors3 Water surveys and designs Survey, design and cost of water projects4 Water from rock outcrops Rock catchment tanks, masonry and earth dams5 Water from dry riverbeds Wells, subsurface dams, weirs and sand dams6 Water from roads Rainwater harvesting from roads7 Water from small dams Ponds and small earth dams built manually8 Water from roofs Various types of roof catchments for domestic useThese handbooks can be obtained free of charge by either collecting them from the office ofASAL Consultants Ltd., or by paying through bank transfer the cost of sending the manuals bycourier. For further details, please contact: asal@wananchi.com with copy toasalconsultants@yahoo.comPublished byASAL Consultants Ltd. forthe Danish International Development Agency (DANIDA) in KenyaText, photos and sketches byErik Nissen-PetersenComputer drawings and tracing byCatherine W. WanjihiaEditing byJoy OkechProofs byAmin Verjee and Steen S. LarsenPrinterEnglish Press, P.O. Box 30127, Nairobi, KenyaWebsite byEdwin OndakoDistribution byASAL Consultants Ltd. P.O. Box 739, Sarit 00606, Nairobi, Kenyaasal@wananchi.com asalconsultants@yahoo.comFax/Tel : 254 (0) 202710296 Mobiles: 0733 619 066 and 0722 599 165Website : www.waterforaridland.com CopyrightThe copyright of this handbook is the property of the Royal Danish Embassy in Kenya.Downloading from the internet and photocopying of the handbooks is permitted providedthe source is acknowledged. i Contents Page
  • Acknowledgement ……………………………………………………… ivAcronyms ………………………………………………………………. vForeword ………………………………………………………………… viChapter 1. Riverbeds ……………………………………………. 11.1 Types of riverbeds ………………………………………………… 11.2 Water storage in sand ……………………………………………… 21.3 Water in riverbeds …………………………………………………. 31.4 Waterholes in riverbeds ……………………………………………. 41.5 Dowsing ……………………………………………………………. 51.6 Evaluation of riverbeds ……………………………………………. 5Chapter 2. Survey of riverbeds ………………………………… 62.1 Probing …………………………………………………………….. 62.2 Probing of Mwiwe riverbed ……………………………………….. 72.3 Survey of Mwiwe riverbed ………………………………………… 82.4 Plan and longitudinal profile of Mwiwe riverbed …………………. 92.5 Cross profiles of Mwiwe riverbed ………………………………… 102.5.1 The most suitable site for the Mwiwe intake ……………………… 102.5.2 The most suitable site for Mwiwe Sand Dam …………………….. 112.6 Trial pits and Survey Report ..…………………………………...... 122.7 Plan and profiles of Nzeeu riverbed ………………………………. 132.7.1 The most suitable site for the Nzeeu intake ……………………….. 142.7.2 The most suitable site for Nzeeu Subsurface Dam ………………… 152.8 Volumes of sand and water in Mwiwe and Nzeeu riverbeds ……… 16Chapter 3. Design considerations ………………………………. 173.1 Sustainability and affordability ……………………………………. 173.2 Water demand …………………………………………………… … 183.3 Existing water yield ………………………………………………… 183.4 Three options for increasing water yield …………………………… 183.5 Design decisions ……………………………………………………. 19Chapter 4. River intakes ………………………………………… 214.1 Suitable seasons for surveys and construction works ……………… 214.2 Intakes in riverbeds ………………………………………………… 214.3 Intakes in riverbanks ……………………………………………….. 234.4 Design and cost of a sinking well …………………………………... 244.5 Design and cost of a hydro-dynamic well head …………………….. 254.6 Water lifts and pumps ………………………………………………. 274.7 Design and cost of large river intakes ………………………………. 294.8 Design and cost of infiltration pipes ……….……………………. …. 304.9 Cost and capacity of pumps, generators and pump houses ………….. 324.10 Calculations and cost of rising mains and head tanks ……………….. 334.11 Map showing pipes, tanks, valves and kiosks of Mbitini Water Project 34 ii
  • Chapter 5. Subsurface dams built of soil ……………….. 355.1 History of subsurface dams ……………………………………… 355.2 Function of subsurface dams ……………………………………. . 355.3 Design and cost of Nzeeu Subsurface Dam ……………………… 365.3 Guidelines on construction ……………………………………… .. 37Chapter 6. Weirs ……………………………………………….. 396.1 Various types of weirs ……………………………………………… 396.2 Design and cost of Talek weir ……..……………………………. … 416.3 Construction procedures …………………………………………….. 436.4 Maintenance of weirs …………………………………………….….. 44Chapter 7. Sand dams ………………………………………….. 457.1 History of sand dams ………………………………………………… 457.2 Functions of sand dams ……………………………………………….. 467.3 Five types of sand dams ………………………………………….….. 477.4 Criteria for construction of successful sand dams …………………... 487.4.1 Site criteria ………………………………………………………. ….. 487.4.2 Design criteria ……………………………………………………….. 517.4.3 Maintenance criteria …………………………………………….. 53Chapter 8. Mwiwe Sand Dam ………………………………... 548.1 Yield of water from Mwiwe Sand Dam ……………………………… 548.2 Design calculations for Mwiwe Sand Dam ………………………….. 548.3 Design of Mwiwe Sand Dam …………….……………………........... 558.4 Construction procedures for Mwiwe Sand Dam ……………..………. 568.5 Bill of quantity and cost of Mwiwe Sand Dam ………………………. 58References ……………………………………………………………. 59 A small riverbed with a subsurface dam built of soil is flooded with rainwater run-off at Kibwezi, Kenya. iii
  • AcknowledgementSincere thanks are due to Birgit Madsen of the Royal Danish Embassy in Kenya who sawthe importance of financing the documentation of how to turn dry riverbeds intoperennial water sources in the semi-desert, arid and semi-arid zones of the world with agrant from the Danish International Development Agency (Danida).Many thanks are also due to the persons whose agencies were interested in the subjectand financed training courses and utilisation of water sources in dry riverbeds. Theseagencies and ministries were; Danida with MoA and MoWI in Kenya from1978 to 2006. Danish Television film Development in 1981. UNDP/ILO/Africare/MoMW in Tanzania from 1990 to1992. UNDP/Habitat in Myanmar (Burma) from 1995 to 1996. Danida and BADC in Kenya in 1996. CONCERN in Somaliland in 1997. Sida and RELMA with MoA and MoWI in Kenya from 1997 to 2002. CONCERN in Southern Sudan 2003. South East Africa Rainwater Network (SearNet) with MoW in Eritrea 2004. Danish Refugee Council (DRC) in Somaliland in 2005. UNDP Somalia in Somaliland in 2005. Royal Danish Embassy in Kenya from 2003 to 2006The successful implementation of the above assignments was mainly due to the support,interest and co-operation of all the engineers, hydro-geologists, technicians, builders andcommunity groups that participated in these activities.Special thanks are due to the team that assisted me in producing this handbook. The teamconsisted of Catherine W. Wanjihia, Civil Engineer Technician, who assisted with thedrawings, Joy Okech, Assisting Information Programme Officer, who edited the draft,James Khamisi who kept the computers working, Amin Verjee and Steen S. Larsen whoproof-read the draft, Prof. Elijah K. Biamah who wrote the Foreword, the English PressLtd. who printed the handbook and Edwin Ondako who loaded the handbook onto theInternet.Erik Nissen-PetersenManaging DirectorASAL Consultants Ltd.P.O. Box 739, Sarit 00606Nairobi, Kenya iv
  • AcronymsASAL = Arid and Semi-arid LandASALCON = ASAL Consultants Ltd.BADC = Belgium Assisted Development CouncilBQ = Bill of Quantitycm = CentimetresCONCERN = A British Non-governmental Organizationcu = Cubic metresDanida = Danish International Development AgencyDRC = Danish Refugee Councilg = gaugeGL = Ground levelHardcore = Crushed stonesILO = International Labour OrganizationITDG = Intermediate Technology Development GroupKm = KilometreKVA = Kilo Volts AmpereKsh = Kenya Shilling equal to US$ 72.00KW = Kilo WattKWSP = Kenya Water and Sanitation Programmem = Metremm = Millimetrem2 = Square metrem3 = Cubic metreMax. = MaximumMoA = Ministry of AgricultureMoWI = Ministry of Water and IrrigationNGO = Non-governmental OrganizationPVC = Poly Vinyl ConduitsRELMA = Regional Land Management UnitRWSS = Rural Water and Sanitation ProgrammeSearNet = South East Africa Rainwater NetworkSida = Swedish International Development AgencyUNDP = United Nations Development ProgrammeUPVC = Ultra Poly Vinyl ConduitsY8 = 8 mm twisted iron barY10 = 10 mm twisted iron barWL = Water level v
  • FOREWORDIn dry land areas of Kenya, episodic shortages of water are quite common. In many areas,women have to spend days traveling long distances with donkeys and camels looking forwater. Where the distances are too long, the people have been compelled to move with theirlivestock to the water sources. Due to the long distances covered searching for water,affected local people have had very limited productive time especially that devoted toagriculture and livestock production. Experiences in dry land areas have showed that whenthe water source is harnessed and conserved through watering points, local people andwomen in particular have more time to concentrate in other household chores and incomegenerating activities.At present in most of Kenya, local people are compelled to live close to permanent watersources; agricultural activities are scattered and localized in transitional agro-climatic zones.Thus it is pertinent that the supply of water for domestic, livestock and supplementalirrigation uses be considered a top priority in managing hydrological and agriculturaldroughts in dry land areas. This supply should be followed by a well planned distributionnetwork of watering points which will meet the water requirements for various uses andminimize environmental degradation due to overgrazing or deforestation. Any developmentor rehabilitation of water supply schemes should aim to ensure reliable and adequate watersupply and sanitation.The socio-economic consequences of inadequate water supply in dry land areas of Kenyaare costly and manifest themselves in low labor productivity, poor enrollment of children inschools, and livestock mortality due to drought and famine. Thus inadequate water supplyultimately decreases per capita water demand, and crop and livestock productivity.Consequently, water shortages reduce household water requirements, household food andwater security and incomes. The low incomes propagate the vicious cycle of poverty andfamine that must be overcome among affected rural communities.The development of appropriate and affordable community water supply systems calls forinnovative rain and runoff water management technologies for domestic, livestock, andsupplemental irrigation uses. Some of the technologies that have proven to be effective andsustainable in water resource development and management in dry land areas include:runoff regulation and storage techniques (e.g. gully head dams, hafirs or waterpans, microdams, ground catchments (djabias), roof catchments, underground water storage tanks (shuijiaos), barkads, and macro and micro catchment systems; and ground water sources such ashand dug wells, shallow drilled wells, infiltration galleries of sand storage dams, and pipedwater from springs. vi
  • Water from dry riverbeds as a hands on manual provides the requisite insights andinformation on how surface runoff that is released into seasonal rivers as flash floods canbe harnessed and abstracted through appropriate technologies such as hand dug wells,weirs and sand storage dams (sand and subsurface dams).This hands-on handbook begins with a historical perspective/background information onsandy riverbeds as reservoirs of good quality water and then goes on to look into thesurvey and design considerations required before constructing hand dug wells, weirs andsand storage dams (sand and subsurface dams). This manual has also addressed theconcern over when to develop these water sources by giving the suitable seasons forsurveys and construction works. The latter information is important because experiencesfrom emergency relief operations in the arid lands of Kenya (i.e. Turkana) have showedthat the construction of sand storage dams in times of drought cannot provide the badlyneeded water to water deficit areas. Instead, these dams should be developed with a viewto being recharged during the rainy season and utilized in subsequent dry spells.Another major contribution by this handbook to the development of water from dry andsandy riverbeds is that of providing the necessary details on design, bills of quantities andcosts, construction and maintenance procedures of the three water supply systems namelyhand dug wells, weirs and sand storage dams. Alongside this, this handbook gives thecosts and capacities of pumps, generator sets and pump houses as well as the calculationsand costs of water conveyance and distribution systems.Indeed the information provided by this handbook will be of immense use to recentlyestablished (under the new Ministry of Water and Irrigation) water resource developmentand management institutions in Kenya such as the Water Resources ManagementAuthority (WRMA), the Water Services Trust Fund (WSTF), the Regional WaterServices Boards, and the grassroots based Water Resources Users Associations(WRUAs).Prof. Elijah K. BiamahChairman/Head,Department of Environmental and Bio-systems Engineering,University of Nairobi,P.O. Box 30197-00100,Nairobi, KenyaEmail: biamahek@yahoo.com vii
  • Chapter 1. Riverbeds1.1 Types of riverbedsDry riverbeds are sandy and seasonalwater courses that transport run-offrainwater from catchment areas, alsocalled watersheds, into rivers or swampsonce or a few times in a year.Dry riverbeds are also called ephemeralstreambeds, seasonal water courses orsand rivers. In Kenya they are called 2) Gullies originating from stony hillsluggah and in Arabic wadi. have a potential for sand dams consisting of medium coarse sand where 250 litresMost of the rainwater being transported of water can be extracted from 1 cu.m.downstream in riverbeds appears as of sand, i.e. an extraction rate of 25%.flash-floods that can be several metreshigh, and that thunder downstream withthe sound of a steam locomotive.Flash-floods can uproot trees and othervegetation growing on riverbanks andfields. Homes, villages and towns mayalso be flooded and people, livestock,buildings and bridges washed away by 3) Riverbeds having catchments of flatthe muddy maelstroms of flash-floods. farmland usually contain fine textured sand, that can only yield a maximum ofRiverbeds may be classified into 4 100 litres of water from 1 cu.m. of sand,classes of potential water sources as i.e. an extraction rate of 10%.follows: 4) Stony riverbeds containing boulders and fractured rocks have the lowest1) The most potential riverbeds have potential for water extraction due tohilly and stony catchments that produce seepage caused by the boulders andcoarse sand where up to 350 litres of fractured rocks. This seepage is,water can be extracted from1 cu.m. of however, beneficial for replenishment ofsand, i.e.an extraction of rate of 35%. boreholes situated on riverbanks. 1
  • 1.2 Water storage in sandSand consists of small stone particles Therefore, much more water can bethat originate from stones and rocks extracted from riverbeds containingbeing broken down by the effects of coarse sand than from riverbeds withsunshine, rains and temperature fine textured sand.variations. No water can be extracted fromVoids, which are empty spaces, are riverbeds containing silt, such as in sandalways found between sand particles. dams whose spillways were built inWhen dry riverbeds are flooded by rains stages higher than 30 cm.and flash-floods, the air in the voids ispressed out by the water because it is Water storage in sand has severalheavier than air. advantages, such as:When a dry riverbed is being flooded, it 1) Evaporation losses are reducedlooks as if the riverbed is boiling as tens gradually to zero when the water level isof thousands of air bubbles are being 60 cm or more below the sand surface.pressed out of the sand. This process isknown as saturation. 2) Livestock and other animals cannot contaminate the water reservoir becauseFine textured sand has tiny voids that it is hidden under a surface of dry sand.get saturated slowly with water. Onlyabout 10% of water can be extracted 3) Mosquitoes, and insects that carryfrom the volume of fine sand. water-borne diseases, cannot breed in underground water reservoirs. Frogs,Coarse textured sand has larger voids snakes and other unpleasant reptiles andand is therefore saturated much quicker animals are also unable to live in, andthan fine sand. The volume of water that pollute, water reservoirs in sand.can be extracted from coarse sand isabout 35% of the volume of sand. Sand testing The porosity and extractable capacity ofSilt and sand extractability was tested sand are found by saturating 20 litres ofand classified as follows: sand with a measured volume of water. Silt Fine Me- Coar- The water is then sand dium se drained out of the sand sand container and Size <0.5 0.5 to 1.0 to 1.5 to measured by mm 1.0 1.5 5.0 removing a plug Satu- from the bottom of ration 38% 40% 41% 45% the container. Water extrac- 5% 19% 25% 35% tion 2
  • 1.3 Water in riverbeds Ficus natalensis Muumo 9 m toSince time immemorial, riverbeds have Muumo 15 mprovided water for people and animals. Ficus Mkuyu 9 m to malatocapra Mukuyu 15 mDuring extreme droughts, when all other Gelia aethiopica Mvungunya 9 m towater sources have dried up, water can Muatini 20 mstill be found in riverbeds. Piptadenia Mganga 9 m to hildebranditi Mukami 20 mElephants, ant-eaters and some other Acacia seyal Mgunga 9 m towild animals have a special sense by Munini 20 mwhich they can locate such places wherewater is found in riverbeds. Since the above mentioned trees must have their tap root in the water table, theSome rural people and most well-diggers depth of a water-table can be found byknow that water can only be found at knowing the depth of the tree’s tap root.certain places in riverbeds. They canalso give a rough estimate on how deep A rule of thumb states that the tap rootthey have to dig before reaching the of a tree has a depth equal to about ¾ ofwater-table. the height of the tree. The height of a tree can be found byTheir knowledge is based on the fact that measuring the length of the shadow thecertain species of trees and vegetation tree is casting on the ground andmust have roots reaching down and into comparing it with the length of thethe water-table in order to survive shadow of a stick 100 centimeters long.droughts. This traditional information iscompiled in the table below. The two measurements should be taken in the sunshine of early morning or late afternoon when the shadows are longest.Water-indicating vegetationBotanical name Kiswahili & Depth Kikamba to names water- levelCyperus 3 m torotundus Kiindiu 7mVangueria Muiru 5 m totomentosa Kikomoa 10 mDelonix elata 5 m to Mwangi 10 mGrewia Itiliku Itiliku 7 m to 10 mMarkhamia Muu 8 m to For example: If the stick’ shadow is 80hildebranditi Chyoo 15 m cm long, the ratio is: 80/100 = 0.8.Hyphaene Kikoko Ilala 9 m tothebacia 15 m If the tree’s shadow is 12 m long, thenBorassus Mvumo 9 m to the tree is: 12 m x 5 / 4 = 15 m high andflabellierfer Kyatha 15 m the tap root and water level is at:Ficus 9 m to 15 m x ¾ = 11.25 m depth.walkefieldii Mombu 15 m 3
  • 1.4 Waterholes in riverbedsThe reason why some waterholes are If riverbeds with impermeable floorsperennial and others are not, lies under have water, then something must havethe sand, where it cannot be observed. stopped the water from being drained downstream. What could that be?1) The floors under the sand inriverbeds can consist of soil, clay, The answer is found by hammering ironmurram, black cotton soil, boulders, rods into the sand of riverbeds at certainfractured rocks or solid rock bars. intervals. Such probing shows that most riverbeds have a floor under the sand2) Where floors consist of permeable that bulges up and down. These natural(water-leaking) material, such as sandy barriers, called dykes, give the answer.soil, fractured rocks or large boulders,water will seep into the undergroundthrough the floor. This may be beneficialfor deep boreholes but certainly not forextracting water from riverbeds.3) If a floor consists of an impermeable(water-tight) texture, such as clay,clayey soil or murram, there is noleakage through the floor. Water can Water is found in this waterhole becausetherefore be extracted from the riverbed, the dyke prevents the water seepingunless it has drained downstream and downwards in the voids between theleft the riverbed dry. sand in the riverbed.4) Riverbeds have downstreamgradients because they function asdrainage channels for rainwater run-off.Water in riverbeds is therefore alwaysmoving downstream, either on thesurface or between the sand particlesunder the surface of riverbeds. Surfaceflow of water can easily be observedduring floods but the subsurface flow Where the floor bulges upwards it actscan only be observed when water is as an underground dyke that stops thescooped out of waterholes. underground flow of water, as seen on probing points no. 10, 13, 15, 19 and 23.The reason why some riverbeds havewater and others have not, lies in the Where there is a depression in the floorfloor under the sand in riverbeds with it accumulates water, as seen on probingimpermeable floors. points no. 9, 14, 16 and 21.If riverbeds have impermeable floors but These are subsurface water reservoirs inno water, it is because the water has the sand, from where water can bebeen drained downstream by gravity. extracted. 4
  • 1.5 DowsingGifted persons can use dowsing to locate 1.6 Evaluation of riverbedsunderground water sources and The most potential stretches of a riverbedunderground dykes. are identified by walking in them together with the community members who have requested for assistance to improve their existing water source or to construct new ones in the riverbed. First draw a sketch of the riverbed.The tool consists of a 1m long brazing Then walk and dowse, if capable of that,rod cut in two halves and each half in the riverbed while plotting thehaving a 12 cm long handle. following information on the map: 1) Location and types of water-indicating trees and vegetation. 2) Location of water-holes and their depth to the water-table and quality of theThe two dowsing rods are held loosely water.and pointing downwards so that they canswing freely. However, the hands must 3) Location and types of rocks andbe held steady to allow gravity to pull boulders.the rods down while they are parallel. 4) Location of calcrete, which is a salty whitish substance that turns water saline. 5) Coarseness of the sand. 6) Location of hand-dug wells, boreholes and weirs in the riverbed. 7) Names of houses, schools and roadHere is water. Here is no water. crossings near the riverbed.When walking slowly over an unknown After having compiled the informationunderground water source, the rods will listed above, the community is informed ofswing inwards. The force of the pull the various possibilities for improvingindicates the depth and volume of water their water supply. When an agreement isin the ground. made with the community, a detailed survey with probing is implemented forAlthough these traditional techniques are the purpose of providing data for drawingrather successful when applied by reliable designs and estimating the yield of water,persons, more precise data is required by and the cost of construction, operation andprofessionals. maintenance. 5
  • Chapter 2. Survey of riverbeds2.1 Probing The tools required for simple surveys asWhen the most promising lengths of a follows:riverbed have been identified during theevaluation walk, they are probed usingprobing rods hammered into the sand. 1) Probing rods made of 16 mm (5/8”) iron rods for measuring depths of sand. Notches should be cut in the probing rods for every 25 cm to collect sand samples when the rods are pulled up. 2) A circular levelling tool made of a transparent hosepipe for measuring the gradients of riverbeds.The probing data is used for:1) Drawing a plan and profiles of theriverbed to identify the deepest place 3) Two long tape measures, onefrom which water should be extracted hanging down vertically from theand the most shallow place where the horizontal one, to measure width andwall for either a subsurface dam, a weir depth of riverbeds.or a sand dam can be constructed. 4) A tripod ladder for hammering long probing rods into the sand.2) Estimating the volume of sand in 5) A mason hammer.the reservoir and the extractable volume 6) A 20 litres jerrycan with water.of water from the sand. 7) Half a dozen of transparent plastic bottles with water.3) Providing the required data for 8) A knife and writing materials,drawing the designs and estimating the 9) The Probing Data Sheets shown oncosts of construction. the next page. 6
  • 2.2 Probing of Mwiwe riverbedParts of the following contents are based on the actual implementation of the Mbitini andKisasi Water Projects situated about 30 km south of Kitui township in Eastern Kenya.The Mwiwe riverbed, which is the water source for the Mbitini project, was surveyed inNovember 2004. The survey started where the Wingo riverbed joins the Mwiwe riverbed.The procedure was as follows:1) A probing rod was hammered down in the middle of the riverbed until it hit the floorunder the sand with a dull sound. Then the level of the sand was marked on the rod and itwas pulled straight up without twisting. The following data was noted on the data sheet:1) The depth of water is measured from the tip of the rod to the water indication mark.2) The depth of sand is measured from the marked sand level to the tip of the rod.3) The coarseness of sand is seen in the notches of the rod.4) The type of floor under the sand is seen at the tip of the rod.5) The width of the sand in the riverbed is measured.6) The height of the banks are measured with two long tape measures.7) The presence of water-indicating vegetation, waterholes, roads, etc. is noted.8) The next probing is measured at regular intervals, for example 20 metres.Probing Data Sheet (m). Location: Mwiwe riverbed Date: 20/11/04Probing Distance Width Depth to Depth Type of Floor Height of Items seennumber between of water of sand sand under the river banks on the probings riverbed sand Left / Right banks 1 0 20.80 No water 0.50 Medium Clay 1.50 / 1.80 Wingo 2 20.00 24.20 No water 0.25 Fine Clay 1.30 / 1.60 3 20.00 28.20 No water 0.28 Medium Clay 1.30 / 1.70 Waterhole 4 20.00 25.50 No water 0.32 Medium Clay 1.42 / 1.84 Pawpaw 5 20.00 23.40 No water 0.45 Coarse Clay 1.30 / 1.65 6 20.00 30.50 No water 0.85 Coarse Rock 1.32 / 1.45 Acacia tree 7 20.00 29.50 No water 0.76 Murram Soft rock 1.32 / 1.50 Rock 8 20.00 33.00 No water 1.00 Coarse Clay 1.97 / 1.55 Waterhole 9 20.00 23.62 0.20 1.25 Medium Clay 0.70 / 1.25 Fig tree 10 20.00 23.62 No water 0.50 Medium Clay 2.25 / 1.67 Tele. Pole 11 20.00 29.60 0.10 1.00 Medium Clay 0.70 / 1.35 Road 12 20.00 32.90 No water 0.59 Medium Clay 0.97 / 1.80 Mukengeka 13 20.00 25.70 No water 0.55 Medium Clay 1.33 / 1.76 Kiindiu 14 20.00 20.00 2.00 3.00 Medium Clay 1.50 / 1.68 Waterholes 15 20.00 17.00 No water 0.75 Medium Clay 1.32 / 1.56 Fence post 16 20.00 26.00 0.10 1.25 Coarse Clay 1.85 / 1.60 Orange tree 17 20.00 22.69 No water 0.93 Coarse Clay 1.00 / 1.45 Orange tree 18 20.00 18.40 No water 0.50 Medium Clay 1.20 / 1.53 Munina 19 20.00 17.50 No water 0.50 Coarse Clay 1.20 / 1.65 Fence post 20 20.00 20.00 0.50 1.75 Coarse Clay 1.46 / 1.58 Mwangi 21 20.00 29.00 0.60 1.75 Coarse Clay 1.75 / 1.67 Fig tree 22 22.00 30.00 No water 0.88 Coarse Clay 1.13 / 1.58 Musewa 23 22.00 26.00 No water 0.45 Coarse Clay 1.20 / 1.45 Kiindiu 7
  • 2.3 Survey of Mwiwe riverbed The horizontal sighting line in the circular levelling instrument hit the pole 0.8 metres above the level of the sand. The difference of 0.8 m in height between probing points No.1 and No. 23 being 440 metres apart, gives a gradient of: 0.8 / 440 m x 100 m = 0.18%. Sand testThe heights and shape of the riverbankswere measured using two long tapemeasures, one hanging vertically downfrom the other which was drawn tightlybetween the top of the riverbanks.The gradient of the riverbed wasmeasured by a person standing atprobing point No. 1 while sightingdownstream using the two horizontal Samples of dry sand were taken andwater levels in a circular hosepipe that filled in a 20 litres container for thewas filled halfway with water. purpose of measuring the porosity and the volume of water that can be extracted from the sand reservoir. The saturation of the sand was reached after adding 8 litres of water, giving a saturation rage of 40% (8 / 20 x 100). Then a small hole was made in the bottom of the container to drain the water out of the sand. In one hour, 5Other persons were standing litres of water was extracted from thedownstream at probing point No. 23 sand. That gives an extractable capacitywhile holding a long pole vertically. of 25% (5 / 20 x 100). 8
  • 2.4 Plan and longitudinal profile of Mwiwe riverbedA plan and a longitudinal profile was drawn according to the probing data on a sheetof A3 mm graph paper which is 40 cm long and 28 cm wide.The horizontal measurements were drawn to a scale 1:2000, which means that the440 metres length of the riverbed was drawn as 22 centimetres long and the 20 metreintervals between the probings were drawn as 1 centimetre long on the graph papers.DOWNSTREAM UPSTREAMPlan of Mwiwe riverbed.The plan shows that the probed river has a length of 440 m and width varying from17.5 m to 33.0 m. Water-indicating trees, e.g. Figs, Mwangi and Munina, grow along theriverbed. Waterholes having water 7 months after the last rains were located at probingpoint No. 10, 14 and 21 where the sand is deep. Water is trapped in the sand bydownstream dykes at points No. 11, 18 and 23, as see on the longitudinal profile below.DOWNSTREAM UPSTREAMLongitudinal profile of Mwiwe riverbed. 9
  • The longitudinal profile on the former page shows that the sand is 3.0 m deep at point No. 14 and1.75 m deep at point No. 21. Since both places are holding water 7 months after the last rain theyare the best extraction points in that part of Mwiwe riverbed. The next phase in surveying theriverbed was to probe across at point No. 17 in order to learn of the volume of the depression andits water yielding capacity.2.5 Cross profiles of Mwiwe riverbed2.5.1 The most suitable site for Mwiwe extraction pointThe riverbed was probed from bank to bank at 2 metre intervals at point No. 14. Toconfirm that point No. 14 was the deepest point, further probing was done both upstreamand downstream from that point. Probing data across the extraction point at No. 14 in Mwiwe riverbed Probing Distance Depth of sand Type of sand Type of floor No. between under the probings m m sand 1 2 0.75 Coarse sand Sandy clay 2 2 3.25 Coarse sand Sand 3 2 2.50 Coarse sand Sandy clay 4 2 1.75 Coarse sand Sandy clay 5 2 3.00 Coarse sand Sand 6 2 1.50 Coarse sand Sand 7 2 0.75 Coarse sand Sandy clay 8 2 NIL Coarse sand Sandy clay LEFT BANK RIGHT BANK Probing profile of No. 14 in Mwiwe riverbed where the sand is deepest and therefore the most suitable place for the intake. 10
  • 2.5.2 The most suitable site for the dam wall of Mwiwe Sand DamThe longitudinal profile and several cross probings showed that the underground dyke atpoint No. 18 was the most suitable for the Mwiwe Sand Dam as shown below. Probing Distance between Depth of Type of sand Type of floor No. probings m sand m under the sand 0 2 0.85 Coarse sand Clay 1 2 1.35 Coarse sand Clay 2 2 1.30 Coarse sand Clay 3 2 1.70 Coarse sand Clay 4 2 1.30 Coarse sand Clay 5 2 1.60 Coarse sand Clay 6 2 1.40 Coarse sand Clay 7 2 1.10 Coarse sand Clay 7.5 0.5 0.40 Coarse sand Clay LEFT BANK RIGHT BANK This cross profile shows the measurements of the most suitable underground dyke at No. 18 for Mwiwe Sand Dam. The design below was based on this profile. Design of Mwiwe Sand Dam that was based on No. 18 cross profile 11
  • 2.6 Trial pitsTrials pits were excavated down to Underground dykes can sometimes be locatedthe floor under the sand every 3 metres by vegetation growing in soil situated justto confirm the correctness of the below the sand surface.probing.The surveyors prepared a Survey Report with all details and data compiled of Mwiweriverbed. The report was handed over to a design engineer for his considerations whichare presented in the next chapter.Thereafter the surveyors surveyed a much larger riverbed, Nzeeu, for 25,000 people inKisasi Water Project. The survey data is presented on the next pages. On the left, a local expert is using a pendulum to find water under the ground. On the right, a soda bottle is used for grading a sand sample 12
  • 2.7 Plan and profiles of Nzeeu riverbedDOWNSTREAM UPSTREAMPlan of Nzeeu riverbedThe surveyors started probing from the junction of Nzeeu and Kindu riverbed at point 0.Thereafter they returned to point 0 and surveyed upstream as shown on these sketches.RIGHT BANK LEFT BANKLongitudinal profile of the probing in Nzeeu riverbed.The plan and profile show a large underground water reservoir at No. 3 and anunderground dyke with a narrow point at probing point No. 13 which is a perfectfoundation for either a subsurface dam or a sand dam. 13
  • 2.7.1 The most suitable site for the Nzeeu extraction pointA cross profile of Nzeeu riverbed at probing point No. 3 on the longitudinal profileconfirms the large underground water reservoir from which water will be extracted. Probing data across the deepest point at No. 3 of Nzeeu riverbed Probing Distance Depth of Type of sand Type of floor No. between sand under the probings m sand m 1 0 0 Fine sand Clay 2 3 2.50 Medum sand Clay 3 3 3.50 Coarse sand Not reached 4 3 4.50+ Coarse sand Not reached 5 3 4.50+ Coarse sand Not reached 6 3 4.50+ Coarse sand Not reached 7 3 4.50+ Coarse sand Not reached 8 3 4.50+ Coarse sand Not reached 9 3 4.50+ Coarse sand Not reached 10 3 4.50+ Coarse sand Not reached 11 3 3.50 Coarse sand Not reached 12 3 2.50 Coarse sand Clay 13 8 2.00 Coarse sand Clay 14 8 2.00 Coarse sand Clay 15 8 2.00 Coarse sand ClayRIGHT BANK LEFT BANKCross profile of Nzeeu riverbed at point No. 3 (A-A) which will be the extraction point. 14
  • 2.7.2 The most suitable site for the dam wall of Nzeeu Subsurface DamA cross profile of the underground dyke and narrow riverbed at point 13 confirms thelocation of the most shallow place for either a subsurface dam, a weir or a sand dam. Probing data across the shallow dyke at No. 13 (B-B) of Nzeeu riverbed Probing Distance Depth of sand Type of sand Type of floor No. between m under the probings m sand 1 0 1.50 Coarse sand Sandy clay 2 3 1.50 Coarse sand Sand 3 3 2.00 Coarse sand Sandy clay 4 3 1.75 Coarse sand Sandy clay 5 3 1.75 Coarse sand Sand 6 3 1.50 Coarse sand Sand 7 3 1.00 Coarse sand Sandy clay RIGHT BANK LEFT BANK Cross profile of Nzeeu riverbed at probing point No. 13 (B-B). The profile was used for the design of the subsurface dam as shown below. 15
  • 2.8 Volumes of sand and water in Mwiwe and Nzeeu riverbedsThe porosity and extractable percentage of water from the sand in Nzeeu riverbed wasfound to be 35% and 30% respectively.The approx. volume of a dam reservoirs can be found by this rule of thumb:Maximum depth x maximum width x maximum throw-back / 6 = Volume.(A through-back is the horizontal length of water in a dam).The volume of sand and water in the reservoirs of the two riverbeds were found to be: Max. depth Max. width Throw-back 1/6 Sand % Water Water m m m Volume m3 extraction volume m3Mwiwe 3.25 25.70 40.00 1/6 557 25 139Nzeeu 5.00 66.00 380.00 1/6 20,900 30 6,270An unknown volume of subsurface flow of water in the sand replenishes the tworeservoirs which should be added to the volume of water shown above.In order to increase the yield of water from Mwiwe riverbed it was decided to construct aa sand dam upon the dyke at No. 18, because that would increase the storage capacityfrom 139 m3 to 2,997m3, as shown under WL 4 in the table below.Three options for increasing the volume of sand and water in Mwiwe riverbed Max. depth Max. width Throw-back 1/6 Sand % Water Water m m m Volume m3 extraction volume m3WL 1 3.25 25.70 40.00 1/6 557 25 139WL 2 0.8 + 3.25 26.00 260.00 1/6 4,563 25 1,141WL 3 1.4 + 3.25 28.00 300.00 1/6 6,510 27 1,758WL 4 2.3 + 3.25 30.00 360.00 1/6 9,990 30 2,997Three options for increasing the volume of sand and water in Nzeeu riverbed Max. depth Max. width Throw-back 1/6 Sand % Water Water m m m Volume m3 extraction volume m3WL 1 5.00 66.00 380.00 1/6 20,900 30 6,270WL 2 0.8 + 5.00 68.00 420.00 1/6 27,608 30 8,282WL 3 1.4 + 5.00 70.00 440.00 1/6 32,853 30 9,856WL 4 2.3 + 5.00 72.00 480.00 1/6 42,048 30 12,614Key:WL 1 = Existing natural reservoir in the riverbedsWL 2 = Increased volume by a subsurface dam wall built to 30 cm below sand levelWL 3 = Increased volume caused by a weir built to 30 cm above sand levelWL 4 = Increased volume caused by a sand dam wall built to 150 cm above sand levelA Survey Report was then given to the engineer to determine whether the dam should bea cheap subsurface dam or a low cost weir or the more expensive sand dam. Theconsiderations for this assessment are described in the next chapter. 16
  • Chapter 3. Design considerations3.1 Sustainability and affordabilitySustainability and affordability are two key words that should always be rememberedwhen designing water projects.Sustainability of water projects can be achieved by designing tested and simplestructures that the community can construct, operate, maintain and repair themselves.For example, initially it was planned to drill two boreholes and equip them withsubmersible pumps at Mbitini and Kisasi water projects at a total cost of about Ksh 3million. Instead, some local builders were contracted to sink two hand-dug wells withinfiltration pipes at a total cost of Ksh 425,000, all inclusive. Not only was theconstruction cost reduced by Ksh 2.5 million but it was much easier and cheaper for thecommunity to service the pumps in the hand-dug wells than it would have been in theboreholes.Affordability is also achieved by designing tested and simple structures using amaximum of locally available skills and materials that the community can afford toprocure, construct, operate, maintain and repair. The example described above withboreholes versus hand-dug wells describes exactly what affordability is all about.The supply capacity of a water project should be able to satisfy a community’s waterdemand for the next 20 years. If the supply capacity is less than the demand, people willcomplain that their water supply is insufficient and they have wasted their money andtime. Where a water project is over-designed, the construction works and operation costsare higher than necessary and the extra cost has to be levied on the sale of water. Thesupply capacity and the water demand should therefore balance.Another factor to consider is the population increase. When water is available, manypeople living outside the project area will be attracted to settle in the project area. Thesupply capacity must also include this population in addition to the normal increase.Water is sold at kiosks from Ksh 2 to Ksh 4 A Kiosk Attendant’s sales record isper 20 litre jerrycan. The price of water checked. The income from sale of waterdepends on the cost of operation, main- must be sufficient to pay for salaries,tenance and repairs. fuel, operation, maintenance and repairs. 17
  • 3.2 Water demandWith reference to an appraisal report produced by Development Impact Consulting (DIC)in August 2004, the combined water demand for Mbitini is projected as shown in thetable below.Daily Water Demand for Mbitini. The water demand by Kisasi is similar to Mbitini.Category m3/day m3/day m3/day m3/day 2004 2005 2015 2025General population 169 172 210 256Livestock units 20 20 23 26Schools 11 11 14 17Health and Adm. Centres 5 5 5 5Shopping centres 12 14 18 21Total daily water demand (m3/day) 217 222 270 325Total annual water demand (m3/year) 80,000 81.000 98,000 118,000Demand in 6 months without rains 40,000 41,000 49,000 59,0003.3 Water yield in year 2004Volume of sand and water in the underground reservoirs Max. depth Max. width Throw-back 1/6 Sand % Water Water m m m volume m3 extraction volume m3Mwiwe 3.25 25.70 40.00 1/6 557 25 139Nzeuu 5.00 66.00 380.00 1/6 20,900 30 6,2703.4 Three options for increasing the water yieldsThe three available options for increasing the water yields are to construct dam walls foreither:1) A subsurface dam built of soil to 30 cm below the surface of sand in a riverbed, or2) a weir built of concrete or rubble stone masonry to 30 cm above the sand level, or3) a sand dam built of concrete or rubble stone masonry to a maximum of 5 metres above the level of sand in a riverbed.These dam walls should be constructed on the identified underground dykes situateddownstream of existing underground water reservoirs.In other riverbeds, the three types of riverbed dams should, preferably, be constructed onunderground dykes. The dams can, however, also be built in riverbeds not havingunderground reservoirs or underground dykes. 18
  • 3.5 Design decisionsThe following decisions on the designs were taken on the basis of the evaluation reportand the considerations described above as well as during several meetings with the Kisasiand Mbitini project committees:3.5.1 Decision on extraction point in Mwiwe riverbedAs mentioned above, it was found more sustainable and affordable to lay infiltrationpipes in the riverbed to drain water into a hand-dug well to be sunk in the riverbank,instead of drilling a borehole and equipping it with a submersible pump.The extraction point will consist of 72 metres of infiltration pipes made of perforated 160mm UPVC laid in crushed stones as deep in the riverbed as the water level would allowat point No. 14 in Mwiwe riverbed. The infiltration pipes will be laid with a gradienttowards the riverbank to facilitate water flowing into a hand-dug well that will be sunk inthe riverbed.The hand-dug well will have a diameter of 3 metres and be sunk as deep as the watertable will allow. The well will be built of curved concrete blocks reinforced together on acircular foundation/cutting ring of reinforced concrete.Extraction of water from the well will consist of a surface pump powered by a dieselgenerator that will be installed in a pump house to be constructed.The pump will deliver the water to a 100 m3 head tank to be constructed of concreteblocks and situated 0.96 km from the pump at an elevation of 80 metres above the pump.From this head tank water will gravitate through 2 water tanks and 22.9 km of pipelinesto 11 water kiosks.3.5.2 Decision on a sand dam in Mwiwe riverbedFor Mwiwe riverbed it was decided to construct a sand dam of rubble stone masonry to aheight of 1.5 metres above the existing sand surface at probing point No. 18. The sanddam will raise the water table from 1.7 metres below the sand level to 1.5 metres above,which is a total of 3.2 metres. This will increase the storage capacity from the existing139 m3 to 2,997 m3 as shown under WL 4 (Water Level 4) in the former chapter.After the infiltration pipes have been laid in the riverbed and the hand-dug well has beensunk as deep as possible, the water table in the riverbed will be raised by construction thesand dam in the required stages. 19
  • 3.5.3 Decision on extraction point in Nzeeu riverbedThe extraction point in Nzeeu riverbed will be a replica of the extraction point in Mwiweriverbed, except for the pump, which will deliver the water to a 50 m3 elevated head tankto be constructed of steel plates. The tank will be situated 5.3 km from the pump at anelevation of 154 metres above the pump. From this head tank water will gravitate through2 water tanks and 16 km of pipelines to 10 water kiosks.3.5.4 Decision on a subsurface dam in Nzeeu riverbedThe selected option for Nzeeu was a subsurface dam whose wall will be built of soil to 30cm below the existing sand surface, which will raise the water table 170 cm. This willincrease the water storage capacity from the existing 6,270 m3 to 8,282 m3.The subsurface dam will have two purposes, namely, a) to increasing the yield of water,and, b) to compare its construction cost and performance with the sand dam at Mwiwe. Survey and design data are being compiled into a Survey Report by some participants of a training course dealing with water from dry riverbeds. 20
  • Chapter 4. Riverbed intakes4.1 Suitable seasons for surveys and construction worksSurveys of riverbeds should always be carried out in the driest seasons when water levelsare at their lowest. Information and mobilization of communities should also beimplemented in the dry seasons when water supplies are scarce and people are concernedabout their water supplies. Designs, bills of quantities, costs and work plans can be drawnup during rainy seasons.Construction of water projects should always start with sinking of the wells and laying ofthe infiltration pipes and that should always take place during a dry season. By doing so,the minimum yield of water can be known and used for calculating the required capacityof the pump and generator, pipelines, water kiosks, etc.The minimum yield is also used to determine whether the riverbed can supply sufficientwater or if a subsurface dam, weir or sand dam has to be built to increase the yield ofwater.If so, the construction work of such structures should take place in the beginning of theshort dry season or in the middle of the long dry season when there is no risk of anunexpected thunder shower that can flood the construction site.Excavation of trenches for pipes, and the laying of pipes, should also take place in the dryseason when the fields have been harvested to avoid spoiling crops in the field. Otheractivities that demand much manual labour should also be implemented when the farmersare less busy in the dry seasons.Another big advantage of constructing water projects during dry seasons is that theprovision of water is usually the biggest issue during those periods. It is therefore fairlyeasy to mobilize and organize people to carry out manual work.More detailed information on survey and design is given in another handbook of thisseries called Water Surveys and Designs.4.2 Intakes in riverbedsThere are several technologies for lifting water from riverbeds that are affordable andsustainable for individuals and community water projects.The oldest and simplest method is to dig awaterhole in a riverbed and use a calabashcut in half to scoop water into a calabash or ajerrycan. 21
  • A safer and cleaner option is to sink a hand-dug well next to the waterhole in a riverbed.Hand-dug wells situated in riverbeds must beequipped with a well-head to prevent the wellshaft being filled with sand during floods suchas this hydro-dynamic well-head used inSudan.Another type of hydro-dynamic well-head isshaped as a wedge to break the force offloods.A well-shaft can also be lined with worn-outlorry tyres and closed by a lid made from thebottom of an oil-drum. 22
  • 4.3 Intakes in riverbanksAlthough sinking of wells directly into riverbeds is the cheapest way to extract water, thewells might be damaged by unusually large flash-floods. It is therefore safer to buildhand-dug wells on the riverbanks and draw water from there provided the undergroundsoil is sandy soil that allows water to seep into the hand-dug wells.If underground water cannot seep into a hand-dug well, an infiltration pipe can be laid in atrench to drain water into the hand-dug well.The design can be made cheaper by filling thetrench with stones covered with polythenesheeting instead of laying the infiltration pipe.Well shafts can be lined with burnt brickswhen built upwards from the bottom of hand-dug wells. However, this procedure is riskybecause collapsing soil can bury the buildersalive.A safer method is to build sinking wellswhereby curved concrete blocks are reinforcedtogether onto a foundation ring made ofconcrete. The foundation ring and infiltrationcourse are built at ground level in riverbeds.Then sand is removed from the inside of theshaft causing it to sink into the sand. When thetop of the shaft has sunk to ground level, moreblocks are added onto the shaft. Sand is thenremoved and the shaft sinks. The procedure isrepeated until the shaft has reached its finaldepth.Concrete culverts can be sunk in a similar waybut they are more difficult to handle due to theirweight.The cost of lining well shafts can be reduced byplacing a large perforated PVC pipe in the centre ofthe excavation and filling stones and gravel allaround it upto the ground level. 23
  • 4.4 Design and cost of a sinking well A tool made of iron for cutting groovesPlan of well-shaft and curved concrete block A groove being cut for a foundation ring with the tool seen above. The groove will be filled with reinforced concrete. Curved concrete blocks are made of mortar in the ratio 1:5 and compacted into a mould made of iron sheets. It takes 16 blocks to make a circular Section of well-shaft course with a diameter of 140 cm and 112 blocks to build 1 metre of shaft. 24
  • 4.5 Design and cost of a hydro-dynamic well headA hydro-dynamic well head can be built around a well shaft directly into riverbeds toprevent the well shaft being damaged or filled with sand from flash-floods.The lay-out of a hydro-dynamic well head Foundation being built of rubblein the middle of a riverbed. stones reinforced with barbed wire.A well head is constructed of rubble stones An open steel cover over the well shaftembedded in mortar, 1:4, and reinforced with should be in the position shown tobarbed wire and chicken mesh. allow flash-floods to close the lid if people have forgotten to close it. A hydro-dynamic well head in the middle of a riverbed. 25
  • Bill of quantity and cost of a well with hydro-dynamic well head, KshDescription Unit Quantity Unit cost Total Value of6 metre deep hand- cost communitydug well with a contributionhydro-dynamicwell headLabour costSupervisor Supervisor 3 days 1,200/day 3,600Contractor Contractor 6 days 800/day 4,800Artisan 1 mason 24 days 200/day 4,800 4,800Trainees 2 trainees 24 days 100/day 4,800 4,800Labourers 4 labourers 24 days 100/day 9,600Cost and value of 18,000 19,200labourMaterialsCement 50 kg bags 26 bags 600 15,600Weldmesh 1.2 m. x 2.4 m 1 sheet 400 400G.I. wire 4 mm 20 kg 50 1,000Chicken mesh 0.9 m x 30 m 4 meters 100 400Barbed wire Gauge 12.5 10 kg 80 800Windlass Two handles 1 unit 3,000 3,000Iron bar Y12 mm 1 length 500 500River sand Tonnes 4 tonnes 200Crushed stones 2.5 mm 2 tonnes 600 1,200 1,200Rubble stones Tonnes 4 tonnes 200 600 600Water 210 litre drums 10 drums 100 1,000Cost and value ofmaterials 23,500 3,600Transport costTrailer loads 3 tonnes 4 loads 900 3,600 1,800Cost of transportTotal cost andvalue 45,100 24,600Total cost 69,700The first part of a well shaft has been builtonto the foundation ring in an excavation dugas deep as safety allows in a riverbed.Insulated electric wires bent as U’s are insertedin the mortar between the blocks in the lowestpart of the well shaft. When the sinking of theshaft has been completed, the wires are pulled out,thereby creating many small holes through whichwater can seep into the well shaft. Also, note thebent iron bars that are mortared into the shaft tofunction as a ladder. 26
  • 4.6 Water lifts and pumps . . A hand-operated pump with a cylinder head made of concrete. Water can be drawn by several types of lifts and pumps. The simplest and cheapest is a bucket tied to a rope. A foot-operated money-maker pump. A windlass made of a few Sisal poles. A direct action pump. A windlass with an iron handle.. A A rope and washer pump. A broken down India Mark II hand pump replaced with a windlass made of Sisal. 27
  • Shallow water pumps (Reference Technical paper No. 13, ITDG). Deep water pumps 28
  • 4.7 Design and cost of large river intakesAccording to the evaluation report, each of the intakes at Mwiwe and Nzeuu riverbeds isrequired to supply 217,000 litres (217 m3) of water daily, which amounts to 80,000 m3annually equal to 40,000 m3 during a 6 month period without flooding of the riverbeds.For each of the two intakes, this yield of water was obtained by sinking a well shaft madeof curved concrete blocks on a foundation ring with an internal diameter of 300 cm. Shortlengths of insulated electric wires were bent into a “U” shape and inserted in each courseof the concrete blocks for every 30 cm with the U facing inwards. The well shaft wassunk as deep as a petrol-powered suction pump could remove the inflowing water.When the capacity of the pump was reached at about 4 metres depth, the internaldiameter of the shaft was reduced to 110 cm in order to reduce the inflow of water. Thewell-diggers then sunk the well shaft to an additional 3 metres before the inflow of waterbecame too much for the diggers and the pump. When the shaft had reached its finaldepth, the insulated bent wires were pulled out of the shaft, each wire leaving two smallinfiltration holes in the shafts for replenishing of the well.A small well shaft being sunk inside a wide The completed well head with thewell shaft of a river intake. pipe from the well to the pump.Plan and section of the intake wells at Mwiwe and Nzeeu (mm). 29
  • Bill of Quantity and cost of the Nzeeu intake wellDescription Unit Quantity Unit cost Total cost Value of8 m deep intake well 2005 2005 community Ksh Ksh contributionLabour cost1 Surveyor Surveyor 2 days 1,200/day 2,4001 Supervisor Supervisor 10 days 1,200/day 12,0001 Contractor Contractor 1 x 20 days 800/day 16,0002 artisans Artisan 2 x 20 days 200/day 8,000 8,0002 trainees Trainees 2 x 20 days 100/day 4,000 4,00010 labourers Labourers 10 x 20 days 100/day 20,000 20,000Cost of labour 62,400 32,000MaterialsCement 50 kg bags 10 600 6,000River sand Tonnes 3 200 600 600Crushed stones Tonnes 3 600 1,800 1,800Curved well blocks Blocks 700 50 35,000 14,000Galvanised wire, 4mm Kg 50 150 7,500Iron bar, Y8 20 m length 10 500 5,000Dewatering pump Days 10 days 800 8,000Cost of materials 63,900 16,400Transport ofmaterialsTractor trailer loads 3 tonnes 1 load 900 900 900Cost of transport 900 900Cost and value 127,200 49,000Total cost 176,2004.8 Design and cost of infiltration pipesIn order to increase the recharge even further, 72 metres of 160mm PVC pipes, intowhich thousands of small holes were burnt with red-hot nails, were connected to theintake. The infiltration pipes were laid in crushed stones as deep as possible and with agradient towards the intake well to enable the water to drain from the sand into the intake.Infiltration pipe connecting to the intake. A flooded trench for infiltration pipes. 30
  • Plan of the infiltration pipes laid deep in the sand of the riverbed.Bill of Quantity and cost of 72 metres of infiltration pipeDescription Unit Quantity Unit cost Total Value ofPerforating and laying 72 cost communitymeters of 160 mm PVC pipe 2005 2005 contributiondeep in a riverbed and slopingtowards a well in the riverbank Ksh Ksh KshLabour cost1 Surveyor Surveyor 4 days 1,200/day 4,8001 Supervisor Supervisor 6 days 1,200/day 7,2001 Contractor with Contractor 1 x 15 days 800/day 12,0002 artisans and Artisans 2 x 15 days 200/day 6,000 6,0004 trainees Trainees 4 x 15 days 100/day 6,000 6,00010 labourers Labourers 10 x 15 days 100/day 15,000 15,000Total cost of labour 51,000 27,000MaterialsDewatering suction pump 8 days 800 6,400Perforated PVC pipes,160 mm 6 m length 14 pipes 3,100 43,400Cost of materials 49,800Transport of materialsTractor trailer loads 3 tonnes 1 load 900 900 450Cost of transport 900 450Cost and value 101,700 27,450Total cost and value 129,150 31
  • 4.9 Cost and capacity of pumps, generators and pump housesThe pump in the Nzeeu pump house is aGrundfoss CR15-17, 15.0 KW, 3 Phasebooster pump, with a capacity of 19m3/hr at154 m head. Together with a control paneland 60A isolator, float switch, chlorinedoser and installation the unit cost wasKsh 560,305 in December 2004.The pump in the photo is a GrundfossCR32-6, 11.0 KW, 3 Phase electric boosterpump with a capacity of 32 m3/hr at 71 mhead, that is installed in the Mwiwe pumphouse. Together with accessories andinstallation the cost was Ksh 558,200 in December 2004.The diesel generator seen in the photo is anAtlas Copco 41, KVA, QUB41 that powersthe Mwiwe pump. The cost was Ksh876,550 in December 2004.The Nzeeu pump is also powered by anAtlas Copco diesel generator with a biggercapacity of 41 KVA, QUB41, that powersthe pump. The cost was Ksh 876,550 inDecember. 2004.The Nzeeu pump house seen in the photo issimilar to the Mwiwe pump house. Bothhouses have a private room for the PumpOperator.The cost of the pump house with a fencewas Ksh 793,866, including the value oflocal labour and materials. 32
  • 4.10 Calculations and cost of rising mains and head tanksThe head tank for the Mbitini Water Projectis a 100 m3 tank built of concrete blocks at acost of Ksh 797,590. The tank is situated ata distance of 0.916 km from the Mwiweintake at a height of 35.1 m above theintake. The pumping head with 100 mm G.Ipipe delivering 30 m3/hr is:Delivery head = 35.1 mFrictional losses = 25.7 mTank height = 3.0 m10% residual (extra) head = 6.4 mRequired pump capacity 70.2 mCapacity of installed pump 71.0 mThe head tank for the Kisasi Water Project isa 50 m3 elevated tank made of steel platescosting Ksh 1,277,621. The tank is situatedat a distance of 5.3 km from the Nzeeuintake and at a height of 74.2 m above theintake. The pumping head, with 100 mm and80 mm G.I. pipes, delivering 19 m3/hr is:Delivery head = 74.200 mFrictional losses = 57.023 mTank height = 8.000 m10% residual (extra) head = 13.922 mRequired pump capacity 153.145 mCapacity of installed pump 154.000 mThe cost of the rising main pipes were: Intake to head tank Km Ksh Nzeuu to steel tank 5.310 7,461,435 Mwiwe to block tank 0.916 1,878,867Water is delivered from the head tanksthrough Distribution pipelines to the waterkiosks by means of gravity, as shown on amap of the Mbitini project area on the next 33 34
  • page.4.11 Map showing pipes, tanks, kiosks and valve chambers of MbitiniWater Project. Mwiwe riverbed 0 1 2 3 4 5 6 7 8 9 10 kmMore technical details and costs on the Mbitini Water Project are given in anotherhandbook of this series: Water Surveys & Designs. 34 23
  • Chapter 5. Subsurface dams built of soil5.1 History of subsurface damsThe oldest subsurface dams known were constructed in the almost waterless area aroundDodoma, in then Tanganyika and now Tanzania, when the railway was built around1905. The purpose of the subsurface dams was to provide water for the steamlocomotives which they successfully did so for many decades.A subsurface dam was built of soil and documented by Bihawana Mission near Dodomain the early 1950s and is still functioning well. A similarly successful subsurface damwas built in the same area in the 1920s. Several subsurface dams were built of variousmaterials, such as soil, burnt bricks, concrete blocks and reinforced concrete during atraining course for ILO in the Dodoma region in 1991. Many more subsurface dams areprobably located in various places, but if so, they are be invisible below the sand surfaceof riverbeds.5.2 Function of subsurface damsShould the water yield from a river intake be insufficient for the demand, then asubsurface dam can be constructed cheaply of soil. The function of a subsurface dam is:a) Block the underground flow of water between the voids in the sandb) Raise the water level in the sand to about 30 cm below the surface of a riverbed.With regard to Nzeeu riverbed, it was decided to construct a subsurface dam of soil thatwould increase the yield from the existing 6,270 m3 of water to 8,282 m3 which, togetherwith the substantial underground replenishment would be sufficient to meet a dry seasondemand of 40,000 m3. Longitudinal profile combined with a three dimensional view of a subsurface dam built of soil, and a hand-dug well built in the deepest part of the riverbed. 35
  • 5.3 Design and cost of Nzeeu Subsurface Dam P l P l Plan and section of Nzeeu subsurface dam (mm) Bill of Quantity and cost of Nzeuu Subsurface Dam Description Unit Quantity Unit cost Total cost Value of 2005 2005 community Ksh Ksh contribution Labour cost Surveyor/designer Surveyor 1 x 2 days 1,200/day 2,400 Supervisor Supervisor 1 x 6 days 1,200/day 7,200 Contractor Contractor 1 x 22 days 800/day 17,600 Artisans Artisans 2 x 20 days 200/day 8,000 8,000 Trainees Trainees 4 x 20 days 100/day 8,000 8,000 Labourers Labourers 10 x 20 days 100/day 8,000 Cost of labour 43,200 24,000 Materials Clayey soil Tonnes 69 100 6,900 6,900 Cost of materials 6,900 6,900 Transport of materials Hiring suction pump Tractor trailer loads 3 tonnes Days 23 loads 900 20,700 10,350 Hiring dewatering pump 4 days 800 3,200 Cost of trans. and pump 23,900 10,350 Cost and value 74,000 41,250 Cost of subsurface dam 115,250 36
  • The sand in a riverbed has been The excavation is being filled with removed and the 60 cm deep key compacted moist clayey soil by has been excavated in the floor. a community in Myanmar (Burma) Two subsurface dams being build by self-help communities in Makueni, Kenya5,3 Guidelines on constructionIn order to get a maximum volume of water for a minimum of work and investment, thewalls of subsurface dams, weirs and sand dams should, whenever possible, always beconstructed on underground dykes that are situated downstream of underground waterreservoirs, identified by waterholes and water-indicating vegetation.When a suitable site has been identified in a riverbed, the most clayey soil forconstruction of the dam wall has to be found. This is done by collecting soil samplesfrom nearby riverbanks and fields. The equipment for analyzing the soil samples consistsof plastic bottles of equal size, of which the caps have been removed and the bottoms cutoff.The bottles are placed upside down in the sand, or sloping against a wall, and filledhalfway with soil samples. Water is poured on top of the soil samples several times. Aftersome minutes it can be observed which soil sample has the slowest infiltration rate due tohaving the highest clay content. This soil is the most suitable for building the dam wall. 37
  • Soil samples being tested for their clay content by pouring water into the soil samples.The sample having the slowest infiltration rate is the most suitable for dam walls.When a suitable site has been identified, all the sand in the riverbed is removed in a 3metre wide stretch between the two riverbanks, so that the floor under the sand is fullyexposed.Thereafter a 100 cm wide trench, called a key, is excavated into the floor right across theriverbed and into the two riverbanks. The depth of the key must be at least 60 cm intosolid soil to prevent seepage under the dam wall.The best clayey soil identified by the testing mentioned above, must now be transportedto the dam site by e.g. sacks, donkeys, ox-carts, wheelbarrows or a tractor with trailer. First the whole length of the key is filled with a 20 cm thick layer of the clayey soil, that is moisten with water and compacted using either tree trunks or cows or a tractor driven forth and back until all air is forced out of the soil. Thereafter, another 20 cm thick layers of soil are laid out along the whole length of the key and dam wall, and moistened and compacted, until the top of the dam wall has reached to 30 cm below the surface of the sand in the riverbed.The upstream and downstream sides of the dam wall, having a slope of about 45 degrees,are then smoothened using shovels and wooden floats. The upstream side of the dam wallshould be plastered with clay or cow dung to prevent water from seeping through the damwall.Finally, the excavated sand is back-filled against both sides and the top of the dam wall.Two short iron bars should be hammered into the riverbanks at each end of the dam wall,to locate the dam wall in the future. 38
  • Chapter 6. Weirs6.1 Various types of weirsWeirs are water-tight structures built across flowing water streams, dry sandy riverbedsand gullies. Only weirs built across dry sandy riverbeds and gullies are described here.Weirs create underground water reservoirs that are recharged with floodwater. They maybecome perennial water sources, provided the storage capacity is sufficiently large andwithout underground leakages. Weirs can be constructed using various designs andmaterials, such as:A wall of soil supported on both sides This long weir was built of rubble stonewith plastic sacks filled with soil, and a masonry across a sandy riverbed. Itspillway at one end of the dam wall. supplies water throughout the year.Rubble stones mortared together with a Once or twice a year, the undergroundspillway at the middle of the dam wall. water reservoir is replenished by floods. A hand-dug well is sunk upstream of thisA similar but more sophisticated weir. weir for extraction of waterThe above weirs were built across The above weirs were built acrossgullies to provide for sand harvesting. riverbeds to supply domestic water. 39
  • This simple and low-cost weirwas constructed across a smallstream by a farmer in CentralKitui. He uses the water forbucket irrigation of vegetablesfor onward sale.Many similar weirs have beenbuilt by farmers. Some of thefarmers use small petrol-powered pumps for irrigationof vegetables and fruit trees.This is a weir constructedacross a small stream onSagalla Hill at Voi, a long timeago.The water flows by gravity to apiped water scheme in thelowlands near the foot ofSagalla Hill.This weir was built across Voiriverbed by Mau Mau prisonersin the 1950s.The purpose was to divert floodwater for large scale seasonalirrigation of farmland situatedat a lower elevation.Water demand was regulatedby opening the gate coveringthe hole seen in the wing wall.Released water would passthrough the hole to a canal inthe field. 40
  • 6.2 Design and cost of weir Plan and profiles of Talek weir (mm). A Longitudinal profile combined with a three-dimensional view of a weir built on an underground dyke and stretching across the riverbed with a hand-dug well situated upstream. A hand-dug well equipped with a stream-lined well head is sunk at the deepest point of the underground water reservoir. 41
  • The Talek weir under construction. PVC pipes were inserted in the dam wall to discharge water accumulating upstream. After completion of the wall, the pipes were removed and the holes in the dam wall were sealed.Bill of Quantity and cost of Talek weir built of rubble stone masonry. Ksh.Description Unit Quantity Unit cost Total cost Value ofA weir with wing 2005 2005 communitywalls having a total contributionlength of 18 metres Ksh KshLabour costA surveyor/designer 1 Surveyor 6 days 1,200/day 7,200A supervisor 1 Supervisor 12 days 1,200/day 14,400A contractor with 1 Contractor 26 days 800/day 20,800his/her team of 3 Artisans 24 days 200/day 14,400 14,400builders and 4 Trainees 24 days 100/day 9,600 9,600community workers 10 Labourers 24 days 100/day 24,000 24,000Cost of labour 90,400 48,000MaterialsCement 50 kg bags 105 bags 600 63,000Barbed wire, g 12.5 25 kg rolls 2 rolls 2,500 5,000River sand Tonnes 15 tonnes 200 3,000Crushed stones Tonnes 15 tonnes 600 9,000Hardcore Tonnes 40 tonnes 200 8,000Water Oil-drums 50 drums 100 5,000Cost of materials 68,000 25,000Material transportTractor trailer loads 3 tonne loads 17 loads 900 15,300Cost of transport 15,300 7,650Cost and value 173,700 50,700Total cost of weir 224,400 42
  • 6.3 Construction proceduresA 60 cm wide trench was dug across the When the trench was filled withriverbed and into the banks of Talek concrete, rubble stones and barbed wireriverbed Maasai Mara in Kenya. at intervals of 30 cm, large flat stones were mortared onto the concrete to formA small stream of water in the riverbed the outside shuttering of the dam wall.was diverted over the trench in PVCpipes. A petrol pump sucked water outof the trench while it was beingexcavated to its final depth of 60 cm intothe solid and impermeable soil. Concrete, rubble stones and lines ofThereafter the trench was filled with barbed wire were filled in between theconcrete packed with rubble stones, and flat stones. The PVC pipes were thenreinforced with double lines of barbed removed and the dam wall plastered.wire, g.12.5. 43
  • 6.4 MaintenanceFlood water passing over spillways always tries to remove the sand from the downstreamside of the weirs. It is therefore always important to place as large boulders as possibleagainst the downstream side of weirs. It might be necessary to use reinforced concrete tobond the boulders together to keep them in place in the event of violent flooding.The photo below shows a weir that was built 20 years ago in the dry area of Mutha. Theweir supplies water all year round for the hand-dug well into the riverbank as seen on thefront cover of this handbook.The boulders that were concreted to the downside of the weir were washed away a longtime ago. The weir and riverbed are now exposed to erosion that may cause the weir to beundermined by flood water. That in turn will result in the weir being washed away,thereby leaving the hand-dug well dry until a new weir is built. 44
  • Chapter 7. Sand dams7.1 History of sand damsSand dams have been constructed and used in India for centuries. The first sand damsknown in Kenya were built by a District Agricultural Officer, Eng. Classen, as part ofa development project named African Land Development Board (ALDEV).That project constructed many earth dams, rock catchment dams, sand dams,boreholes and rangeland schemes in Ukambani from 1954 to independence in 1963. Some of the sand dams built 50 years ago are still functioning, despite the lack of maintenance. An example of the successful sand dams built in the 1950’s is Manzui at Kyuso.Some 200 sand dams have been constructed in Machakos, Makueni, Kitui, Mwingi,Embu and Meru by various agencies and ministries since the early 1970s. Anevaluation showed that only about 5% of the dams built during the last 40 years werefunctioning. This high failure rate is not surprising, since younger engineers do notlearn about subsurface dams, weirs and sand dams.This handbook is based on the techniques used for survey, design and construction ofsand dams built according to the ALDEV design in Tanzania, Burma, Somalia,Eritrea and Kenya since 1978.For additional information on survey, design, construction, etc.of sand dams, pleaseturn to the References on the last page.Hopefully, this will assist relevant agencies and ministries to realise the big potentialfor turning some of the thousands of dry riverbeds in the arid and semi-desert regionsof Kenya into perennial water sources for people, livestock and irrigation. 45
  • 7.2 Functions of sand damsWater supplyThe main function of sand damsis to increase the volume of sandand water in riverbeds.The Mwiwe sand dam increasedthe volume of extractable waterfrom139 m3 to 2,997 m3, althoughthe fact that the reservoir wasonly recharged once by a smallrain shower during the last 18months. 75 m3 of fresh water isextracted daily from the riverbed.Sand harvesting and rehabilitation of gulliesSand dams can also be built ingullies to supply both sand andwater, while also rehabilitatingthe gullies. 10 sand dams built ingullies harvest sand instead of thesand silting up Lake Victoria. Theconstruction cost of the sanddams was recovered in less than18 months through the sale ofsand.If the purpose of building sanddams is to harvest sand andrehabilitate gullies, then weirs builtof plastic bags filled with soilare more profitable.Riverbed crossingsRural roads often cross riverbeds.In remote areas, people simplydrive across such riverbeds.Passable crossings can be madeby concreting a curved andreinforced concrete slab over theriverbed called “Irish Bridges” byhumorous Englishmen.Riverbed crossings can be madeinto sand dams capable of holdingwater upstream of the crossing, asseen in this photo from Mulangoat Kitui, where SASOL hasconstructed about 480 weirs. 46
  • 7.3 Four types of sand damsThis photo is of a sand dam builtaccording to the ALDEV designwhich has functioned for 50 yearswithout failure. 35% of water can beextracted from its sand reservoir, ifthe spillway is raised in stages of 30cm height above the level of sanddeposited by floods. The designcriteria and construction procedureare explained on the following pages.The spillway of this sand dam inTharaka, Meru, was not built instages. Therefore its reservoircontains fine-textured sand and silt,and hardly any water can beextracted.The sand reservoir of this sand damin Taveta was also silted up with siltand fine-textured sand, due to thespillway was not built in stages.Therefore no water can be extractedfrom this dam.USAID funds built this interestingsand dam in Kitui in the 1980s. Thedam reservoir contains coarse sandfrom which people draw water.Although the dam has two spillways,one made of steps and being higherthan the other without steps, the leftside of the spillway is heavily erodedby floods. 47
  • 7.4 Criteria for construction of successful sand damsThe major reasons for many failed sand dams, are caused by insufficient knowledgeof:a) Identifying suitable sites for dam walls and dam reservoirs.b) Design criteria and flood dynamics.c) Construction procedures.d) Maintenance requirements.Should a specific site not meet all the criteria shown on the following pages,construction work should not be initiated.7.4.1 Site criteria1) Suitable riverbeds must have twohigh riverbanks, to enable the wing wallsto keep over-flowing flood water withinthe spillway, and not flowing over theriverbanks.If flood water is allowed to flow over thewing walls and riverbanks, it will erodethe riverbanks and cause the river tochange its course, thereby leaving the sanddam as a ruin in the riverbank, as seenhere.2) Dam walls should never be built onfractured rocks or large boulders becausesuch walls cannot be made water-tight.Water will always seep out between theboulders, as seen in this photo.3) Dam walls should therefore always bebuilt either on a solid bedrock base, orkeyed 1 metre into solid and impermeablesoil. If dam walls are keyed less than1 metre into solid and impermeable soil,water will find its way out under the damwall, and cause the wall to hang in itswing walls over the riverbed. 48
  • 4) Suitable sites for dam reservoirsshould be without boulders andfractured rocks, because they willdrain water from the reservoir into theground below.While such seepage is unwanted forsand dams it can be beneficial forrecharge of ground water and nearbyboreholes.5) Riverbeds with fine-textured sandoriginating from flat land, are alsounsuitable for sand dams, because lessthan 5% of the water stored in thevoids between the sand particles canbe extracted.6) Wide riverbeds, say of more than25 metres width, are also unsuitablefor sand dams, because thereinforcement required for the longdam walls is too expensive.Riverbeds exceeding 25 metres inwidth are suitable for subsurface damsbuilt of soil, because that material ispliable and does not crack likeconcrete.7) The whitish stones seen on theright side of the photo are calledcalcrete. Calcrete is licked bylivestock and wild animals, because itcontains salt and other minerals.If calcrete is situated in the riverbanksupstream of a dam, then the water willbe saline and therefore only useful forlivestock. 49
  • 8) Vegetation that indicate thepresence of water, should be growing onthe banks where the reservoir will belocated, as proof of the riverbedcapacity to store water.A list of water-indicating vegetation,such as the Mukuyu (wild fig) seen nextto a hand-dug well in a dry riverbed inthe photo.9) Waterholes, even temporary ones,should preferably be located wheredam reservoirs are to be constructed toprove that the riverbed has no leakagesdraining water into the ground below.10) Catchment areas should containstones or stony hills, from whererainwater and floods can transporteroded stone particles of which coarsesand is made to the riverbeds,and deposit them into the dam reservoirs.11) As mentioned earlier, the walls ofsand dams, weirs and subsurface damsshould always be constructed on naturalunderground dykes, to benefit fromstorage of water while also reducingconstruction costs. 50
  • 7.4.2 Design criteria1) The most important design criteria isto ensure that flood water will depositcoarse sand into the dam reservoir fromwhere up to 35% of water can be extracted.Many reservoirs contain silt and finetextured sand from where little or no watercan be extracted as seen in the photo.Flood water contains all sizes of silt andsand particles. Since sand is heavier thansilt, sand will always be transported in thelowest part of the current of the flood water,while silt being lighter will float in theupper part.Therefore, if flood water passes over abarrier about 30 cm high, the barrier willtrap the heavy coarse sand, while the lightersilt will pass over the barrier on its waydownstream to the riverbed.In practice, the harvesting of coarse sand isachieved by raising the spillway of sanddams in stages of 30 cm height over thesand level in a riverbed.When flood water has deposited coarsesand to the first stage of 30 cm, the spillwayis again raised a further 30 cm. When thesand from next flood has reached the heightof the second stage, the spillway is againraised by yet another stage of 30 cm and soon until the final height of the spillway isreached as seen in the opposite sketch.The photo shows the spillway of a sanddam at Makueni, that has reached thesecond 30 cm stage of raising the spillway..Judging from the height of the personstanding on the spillway, another 6 stages,each of 30 cm, are to be construced whenthe next 6 floodings have deposited theircoarse sand. 51
  • 2) The force of the water and sand in damreservoirs against the wall of the sand damsis counter balanced by making the widthof the base for dam walls 0.75 (3/4) ofthe height of the dam wall.3) As mentioned earlier, dam walls mustbe keyed at least 1 meter into solid andimpermeable soil. The thickness of the keyshould be 0.55 of the height of its dam wall.4) The width of the crest and its height onthe downstream side should be 0.2 (1/5) ofthe height of its dam wall.5) In the ALDEV design shown here thefront of the dam wall is leaning downstreamwith a gradient of 0.125 (1/8) of the height ofthe dam wall.6) The spill-over apron ontowhich the over-flowing waterwill fall with force, must bereinforced onto the dam wall.The apron must be of thesame width as the dam walland extend up along thewing walls.Large stones should be setinto the concrete to break theforce of water spilling over.7) The key under the dam wall mustextend all the way up to the end of thewing walls, otherwise water might seepunderneath them, thereby eroding the wingwalls and riverbanks.8) Water can be extracted from sanddams by any one of the types of riverintakes mentioned earlier, or througha galvanised pipe inserted into the damwall to a tapping point somewheredownstream. 52
  • 7.4.3 Maintenance criteriaSand dams require careful maintenance, and immediate repair, if necessary, asflooding causes hundreds of tonnes of water to fall over the dam wall and onto thespill-over apron. Flood water may also spill over and erode the wing walls and,perhaps, even over the riverbanks during heavy rains.Owners of private sand dams can usually mobilize repair works quickly and keeptheir sand dams functioning well for many years. Maintenance and repair ofcommunity-owned sand dams take much longer, because the committees have to meetand decide what to do and how to pay for it.This sand dam was surveyed, designedand constructed successfully accordingto the laid down criteria during a trainingcourse at Wote in Makueni in 1996.During a study tour to the sand dam acouple of years later, the community wasadvised to repair the eroded apron at thewing wall. It didn’t. The dam wall waswashed away during the next flood.The lower part of the spill-over apronwas washed away by heavy floods, dueto insufficient reinforcement.If the repair had not been carried outpromptly, water would have seepedunder the dam wall and destroyed it.This sand dam at Kibwezi has a numberof problems, due to having been built onlarge boulders, and complete lack ofmaintenance since it was built by MoA in1978.Despite all the criteria listed above, sanddams have a great potential for supplyingwater from small riverbeds in the semi-arid, arid and semi-desert zones ofAfrica, provided the criteria are adhered to. 53
  • Chapter 8. Mwiwe Sand Dam8.1 Yield of water from Mwiwe Sand DamAs mentioned earlier, Mwiwe sand dam was constructed to increase the volume of thewater reservoir from 139 m3 to 2,997 m3 from a small riverbed, Mwiwe, in Kitui.Although the spillway was built only to the second stage,i.e. 60 cm above the originalsand level, 75 m3 of fresh water was pumped from the reservoir every day for thefollowing 18 months, amounting to a total 40,500 m3 . The recharge came from a smallflooding, and a small continuous underground flow of seepage from some weirs situateda few kilometres upstream in the riverbed.When more floods have deposited sand in the reservoir the spillway will be raised instages, each of 30 cm height.8.2 Design calculations for Mwiwe Sand DamThe Mwiwe Sand Dam was designed according to the specifications of the ALDEVdesign, that has proven to be successful since the mid 1950s. The main specifications ofthis particular design are:The height of the spillway is determined by the maximum height of floodwater that cansafely spill over the spillway without eroding the upper ends of the wing walls and theriverbanks. The maximum height of floodwater can usually be obtained by interviewingelderly residents who can point out how high the water level had reached during thehighest flooding in their time.In Mwiwe riverbed, the highest flood level was said to be 100 cm above the present sandlevel. Since the two riverbanks were 250 cm above the same sand level, the maximumheight of the spillway was found to be: Height of riverbanks: 250 cm Highest flood level: 100 cm Maximum height of spillway: 150 cm 54
  • The Mwiwe dam wall has the required criteria of: a) A base that is 0.75 (3/4) of the height of the spillway equal to: height 150 cm x 0.75 = 112.5 = 113 cm. The key under the dam wall has a depth of: 100 cm into solid and impermeable soil, and a width of: height of dam wall x 0.55 equal to 150 cm x 0.55 = 82.5 cm = 83 cm b) The upstream side of the dam wall has a gradient of: height x 0.125, equal to:150 cm x 0.125 = 18.75 cm = 19 cm. c) The crest has a width of: Height x 0.20, equal to 150 cm x 0.20 = 30 cm and is vertical on the downstream side, for a similar length of 30 cm. d) The spillway was only raised in stages 30 cm above the level of sand in the reservoir.The keys under the two wing walls have: A width of 45 cm and a depth of 60 cm into solid and impermeable soilThe spill-over apron is: Reinforced together with the base of the dam wall. Has a width being equal to the base = 113 cm. Stretches all along the downstream side of the dam wall and wing walls with large rubble stones mortared in the concrete.8.3 Design of Mwiwe Sand Dam 55
  • 8.4 Construction procedures for Mwiwe Sand Dam1) The outline of the wing walls, dam wall andspill-over apron, was marked with wooden pegsand nylon strings according to the measurementsgiven on the design criteria.2) All top soil, sandy soil and roots within themarked areas was removed until solid andimpermeable soil was reached.3) The key and base were excavated into the solidand impermeable soil, under any layer of sandysoil, as follows:Structure of sand dam Depth WidthBase of dam wall 30 cm 113 cmBase of spill-over apron 30 cm 113 cmKey under dam wall 100 cm 83 cmKey under wing walls 60 cm 45 cm4) Two templates were made of timber. Theinner sides of the templates gave the outline of thespillway and dam wall, as in the design criteria.The height of the wall was 150 cm. The upstreamwall had a gradient of 19 cm to the vertical. Thecrest was 30 cm wide. The downstream wall had avertical drop of 30 cm, followed by a slopetowards the base of 113 cm.5) The completed excavation was then filledwith 30 cm high layers of cement mortar, ofmixture 1:4, into which many boulders werecompacted, but without touching each other.6) The rubble stone masonry was reinforcedwith Y8 twisted iron bars laid in the concrete,for every 30 cm height.7) After the excavation was filled with rubblestone masonry, the two templates were erected atthe ends of the spillway for the purpose of givingthe outline of the dam wall, spillway and wingwall. 56
  • 8) Nylon strings were drawn tightly from theinner corners of the templates to pegshammered into the soil next to the upper endof the wing walls to determine the position ofthe outer sides of the masonry wall.9) Flat stones were then set in cement mortar1:4 along the inner lines of the strings.10) Next day, the space between the flatstones was filled with mortar, 1:4, into whichround rubble stones were compacted.Thereafter, flat stones were mortared onto thewing walls so that they could be filled withmortar and stones next day.11) The base of the dam wall, the spill-overapron and the spillway,(the latter beingsituated between the two templates), wereonly raised to 30 cm above the original sandlevel in the riverbed.12) A small flooding deposited a 30 cmlayer of coarse sand that reached the first stageof the spillway. The spillway was thereforeraised another 30 cm above the sand level,for the next stage of the spillway.13) Large boulders were concreted into thespill-over apron, to reduce the velocity (speed)and speed of surplus water falling over thespillway and wing walls.14) The wing walls were thereafter plastered,while the spillway was left open until the nextflooding.15) The next flooding deposited coarse sandup to the level of the spillway. The spillwaywas raised another 30 cm above the new sandlevel.The process of rising a spillway in stages of30 cm height, may be completed in one rainyseason provided the required number offlooding occur and builders are ready fortheir work without delay. 57
  • 8.5 Bill of quantity and cost of Mwiwe Sand DamDescription Unit Quantity for Unit cost Total cost Value ofA sand dam being 2 m high and 12 Mwiwe sand 2005 2005 communitym long dam Ksh Ksh contributionLabour costA surveyor/designer 1 Surveyor 10 days 1,200/day 12,000A supervisor 1 Supervisor 18 days 1,200/day 21,600A contractor, including 1 Contractor 1 x 42 days 800/day 33,600team of builders and 3 Artisans 3 x 40 days 200/day 24,000 24,000community workers 4 Trainees 4 x 40 days 100/day 12,000 12,000 10 Labourers 10 x 40 days 100/day 40,000 40,000Cost of labour 143,200 76,000MaterialsBags of cement 50 kg bags 245 600 147,000River sand Tonnes 150 200 30,000Hardcore 2" to 6" Tonnes 120 200 24,000Water Oil-drum 30 100 3,000Barbed wire, g 12.5 25 kg rolls 4 3,000 12,000 1,500Y 8 twisted iron bars Length 30 350 10,500Cost of materials 169,500 58,500Transport of materialsHardware lorry loads 7 tonne 2 loads 5,000 10,000Tractor trailer loads 3 tonnes 44 loads 900 39,600 19,800Cost of transport 49,600 19,800Cost and value 362,300 154,300Total cost of Sand Dam 516,600Bill of Quantity and cost of closing the spillwayDescription Unit Quantity for Unit cost Total cost Value ofClosing last 3 stages of 30 cm closing 2005 2005 communityheight of spillway spillway Ksh Ksh contributionLabour costSupervisor 1 Supervisor 2 days 1,200/day 2,400Contractor, inclusive his/her 1 Contractor 1 x 3 days 800/day 2,400team of builders and community 1 Artisan 1 x 6 days 200/day 1,200 1,200workers 2 Trainees 2 x 6 days 100/day 1,200 1,200 10 Labourers 10 x 6 days 100/day 6,000 6,000Cost of labour 13,200 8,400MaterialsBags of cement 50 kg bags 20 600 12,000River sand Tonnes 4 200 800Hardcore 2" to 6" Tonnes 10 200 2,000Water Oil-drum 15 100 1,500Cost of materials 12,000 4,300Transport of materialsHardware lorry loads 3 tonne 1 load 3,000 3,000Tractor trailer loads 3 tonnes 3 loads 900 2,700 1,350Cost of transport 5,700 1,350Cost and value 30,900 14,050Total cost of spillway 44,950Total cost of Mwiwe Sand DamSand dam with open spillway 362,300 154,300Completing spillway in 3 stages 30,900 14,050Cost and value of sand dam with spillway 393,200 168,350Grand total of sand dam with completed spillway 561,550 58
  • ReferencesAgarwal, A. and Narain, S. 1991. Dying Wisdom.Centre for Science and Environment, India.Ahnfors, O. 1980. Groundwater arresting sub-surface structures. Sida/ Govt. of India.Backman, A. and Isakson. 1994. Storm water management in Kanye,BotwanaBurger, S.W. 1970. Sand storage dams for water conservation. Water Year 1970. S. Africa.Faillace, C. and E.R. 1987. Water Quality Data Book. Water Development Agency, Somalia.Fewster, E. 1999. The feasibility of Sand Dams in Turkana. Loughborough University, UK.Finkel & Finkel Ltd. 1978. Underground dams in arid zone riverbeds. Haifa, Israel.Finkel & Finkel Ltd. 1978. Underground water storage in Iran. Haifa. Israel.Gould, J. and Nissen-Petersen, E.1999. Rainwater Catchment Systems. IT Publications, UK.Hudson, N.W. 1975. Field Engineering for agricultural development. Oxford, UK.Longland, F. 1938. Field Engineering. Tanganyika.Newcomb, R.C. 1961. Storage of groundwater behind sub-surface dams. US Geol.Survey. USNilsson, A. 1988. Groundwater Dams for small-scale water supply. IT Publications. UK.Nissen-Petersen, E. 1982. Rain Catchment and Water Supply in Rural Africa. Hodder/Stoughton.Nissen-Petersen, E. and Lee, M. 1990. Subsurface dams and sand dams. No. 5. Danida Kenya.Nissen-Petersen, E. 1995. Subsurface and Sand-storage Dams. UNDP/Africare, Tanzania.Nissen-Petersen, E. 1990. Harvesting rainwater in semi-arid Africa. Danida, Kenya.Nissen-Petersen, E. 1996. Ground Water Dams in Sand-rivers. UNCHS, Myanmar.Nissen-Petersen, E. 2000. Water from Sand Rivers. RELMA/Sida, Kenya.Mutiso, G. and Thomas, D. 2000. Where there is no water. SASOL, Kenya.Raju, K.C.B. 1983. Subsurface dams and theirs advantages.Ground Water Board, India.Sandstrom, K. 1997. Ephemeral rivers in the tropics. Linkoping University, Sweden.Slichter. 1902. Sub-surface dams.USGS Water Supply and Irrigation. USA.Wipplinger, O. 1958. The storage of water in sand. Water Affairs, South West Africa.Wipplinger O. 1974. Sand storage dams in South-West Africa. South Africa. 59
  • Training, implementation and documentation ofrainwater harvesting in ASAL regionsIntroduction Services offeredASAL Consultants Ltd. promotes ASAL Consultants Ltd. offeraffordable water in dryland using technical services on:locally available skills and * Ponds and earth dams.materials to construct affordable * Subsurface and sand dams.and sustainable water projects * Hand-dug wells, river intakes.together with communities in * Roof, rock, road and groundASAL (Arid and Semi-arid catchment systems.Land). A hydro-dynamic well-head on a deep hand-dug well in a lugga.Rock catchment tank and dams.An earth dam built manually. A subsurface dam being constructed of rubble stone masonry.We are specialised in ASAL Consultants Ltd. wasimplementing a combination of registered in Kenya in April 1990practical and theoretical training as a consulting firm.courses for engineers, techniciansand artisans. Erik Nissen-Petersen ASAL Consultants Ltd.The participants learn hands-on P.O.Box 739, Sarit 00606,how to survey, design and Nairobi,Kenyaconstruct water projects in co- asalconsultants@yahoo.comoperation with self-help groups. asal@wananchi.com Tel. 0733 619066 & 02 2710296 60